The concept of an internal combustion engine with a variable compression ratio (VCR) has existed for more than 100 years. Probably the earliest U.S. Patent on a VCR was No. 651,966 by Fleury, issued in 1900. Since then, over 70 U.S. Patents have been issued on engines with VCR systems or on VCR mechanisms. In 2000, Saab displayed the SVC (Saab variable compression) engine in the Geneva auto show, and since then VCR has attracted enormous attention.
The VCR engine displayed by Saab divides the engine into two parts—engine head and crankcase section (U.S. Pat. No. 5,443,043 by Nilsson et al.). The engine head includes the piston cylinder block, and the crankcase includes a crankshaft. The engine is capable of tilting its head while keeping the crankcase straight up. Tilting of the head causes a change in cylinder volume, but the change in cylinder volume is most pronounced when the volume is minimum, and thus the compression ratio changes.
A VCR engine of different design by Ehrlich (U.S. Pat. No. 6,202,623 B1) uses modified crank pin design. In Ehrlich's engine, the metal member that is used as a bearing of the crank pin has two holes (one for the connecting rod pin bearing and the other for the crank pin bearing) and the trajectory of the rotational axis of the crank pin can be changed by a handle that is affixed to the metal member that holds the crank pin bearing.
A VCR engine of another design by Yapici (U.S. Pat. No. 6,588,384) uses eccentric rings that support the crankshaft. The crankshaft can be moved up and down by rotating the eccentric rings. The engine's rotational force is outputted through concentric inner gear affixed to the flywheel. The invention by Yapici teaches that an engine equipped with his VCR mechanism does not require significant modification of the engine.
These VCR mechanisms, however, have weaknesses also. In the engine invented by Nilsson et al., the connection of the engine with the exhaust system must be made flexible enough to absorb the continuous movement of the engine if the exhaust system is kept stationary. In the engine invented by Ehrlich, the VCR mechanism adds extra inertia and friction-causing parts, and thus frictional loss must increase, especially at high-speed operation. In the engine by Yapici, the spur gear teeth of the eccentric rings must bear the force due to the reciprocating movements of the piston.